It is conceivable that some of the apparent 'gaps' in the fossil record really do reflect sudden change in a single generation. It is conceivable that there really never were any intermediates; conceivable that large evolutionary changes took place in a single generation. A son might be born so different from his father that he properly belongs in a different species from his father. He would be a mutant individual, and the mutation would be such a large one that we should refer to it as a macromutation. Theories of evolution that depend upon macromutation are called 'saltation' theories, from saltus, the Latin for 'jump'.
Macromutations - mutations of large effect - undoubtedly occur. What is at issue is not whether they occur but whether they play a role in evolution; whether, in other words, they are incorporated into the gene pool of a species, or whether, on the contrary, they are always eliminated by natural selection. A famous example of a macromutation is 'antennapaedia' in fruitflies. In a normal insect the antennae have something in common with the legs, and they develop in the embryo in a similar way. But the differences are striking as well, and the two sorts of limb are used for very different purposes: the legs for walking; the antennae for feeling, smelling and otherwise sensing things. Antennapaedic flies are freaks in which the antennae develop just like legs. Or, another way of putting it, they are flies that have no antennae but an extra pair of legs, growing out of the sockets where the antennae ought to be. This is a true mutation in that it results from an error in the copying of DNA. And it breeds true if antennapaedic flies are cosseted in the laboratory so that they survive long enough to breed at all. They would not survive long in the wild, as their movements are clumsy and their vital senses are impaired.
So, macromutations do happen. But do they play a role in evolution? People called saltationists believe that macromutations are a means by which major jumps in evolution could take place in a single generation. If saltationism were true, apparent 'gaps' in the fossil record needn't be gaps at all. For example, a saltationist might believe that the transition from sloping-browed Australopithecus to dome-browed Homo sapiens took place in a single macromutational step, in a single generation. The difference in form between the two species is probably less than the difference between a normal and an antennapaedic fruitfly, and it is theoretically conceivable that the first Homo sapiens was a freak child - probably an ostracised and persecuted one - of two normal Australopithecus parents.
